Current trends in gas turbine engines are moving towards so-called geared turbofan engines in which the fan is driven through a reduction gear train. The gear train allows the low pressure spool to be driven at higher rotational speeds, which provides for a more efficient lighter engine core, whilst reducing the speed of the fan, which allows it to have a larger diameter and enable a higher bypass ratio. The reduction gear trains may be epicyclically configured where the fan is driven via the carrier of a planetary configuration, or a star configuration where the planet gears are fixed and the fan shaft is driven by the ring or star gear.
FIG. 1 shows a geared gas turbine engine 10 having a fan 12, low and high pressure spools, each having respective compressors and turbines driveably interconnected by respective shafts which are rotatable about a principal axis. Thus, there is a low pressure compressor 15 connected to the low pressure turbine 19 via a low pressure shaft, and a high pressure compressor 16 connected to a high pressure turbine 18 via a high pressure shaft. The low 15 and high 16 pressure compressors progressively compress air from an inlet downstream of a fan 12 to an outlet in flow proximity to the combustor 17. Compressed air flows from the high pressure compressor 16 to the combustor 17 in which fuel is added and the mixture burnt. The combusted gas then expands through and drives the high 18 and low 19 pressure turbines in flow series. The low and high pressure shafts interconnect the respective turbines and compressors to provide the drive for the compressors.
The fan 12 is located at the front of the engine 10 to provide air for the inlet of the compressors and the main propulsive flow which is channelled down the bypass duct 22. The fan 12 is driveably connected to the low pressure shaft via a gear train 14 in the form of an epicyclic reduction gear box. The gear train 14 is located between the low pressure shaft and the fan 12 and is arranged to reduce the speed of the fan 12 relative to the speed of the low pressure turbine 19. Such an arrangement allows for a higher speed and more efficient low pressure turbine 19 together with a slow spinning larger fan which can provide a higher bypass ratio. This combination allows the speed of the fan and low pressure turbine to be independently optimised.
The fan 12 has a plurality fan blades 13 extending radially from a hub which is mounted so as to rotate about the principal axis of the engine 10. The fan 12 resides within a fan casing 21 which partially defines the bypass duct 22. An engine casing surrounds the engine core which comprises the low and high pressure spools and combustor 17. The engine casing generally provides containment and structural support for the engine core. The engine casing is ultimately attached to and supported by the wing of the aircraft via an appropriate arrangement of struts which extend across the bypass duct and the nacelle which attaches to a pylon as is well known in the art.
The gear train 14 is in the form of an epicyclic reduction gearbox which is driven in a planetary configuration. The gear train 14 includes a ring or annular gear which is heldsubstantially stationary in relation to the engine casing, a planet gear set with individual planets gears interconnected via a carrier, and a sun gear. The sun gear is rotatably connected to the low pressure shaft. The fan 12 is connected to the output shaft of the gearbox which is in the form of the carrier of the planet gear via a fan shafting arrangement.
Generally, planetary gearboxes are used in power transmission systems across many industries including, for example: automotive, wind turbines, aerospace and marine. In its simplest form, it comprises a central gear or sun gear surrounded by multiple planet gears mounted on a single concentric carrier, which in turn sits within a single concentric ring gear which has internal gear teeth for engagement with the planets.
In operation, one of the sun gear, planet carrier and ring gear are held stationary with the other two providing an input and an output to the gearbox. The selection of the stator, input and output determines the gear ratio of the gearbox and allows for several drive variations, as are known in the art.
A common variation for high power density gear boxes is to provide a stationary ring gear, a sun gear input with the planet gear carrier driving the output shaft. When using such a configuration, the stationary ring gear should be constrained in such a way that it can accommodate build tolerances and avoid deleterious stress on the supporting structures, gears and bearings, in service.
The present invention seeks to provide an improved ring gear mount for a planetary gearbox.